MICRF220 Ver la hoja de datos (PDF) - Micrel

Número de pieza

componentes Descripción

Fabricante

MICRF220

300MHz to 450MHz, 3.3V ASK/OOK Receiver with RSSI and Squelch

Micrel 

Micrel, Inc.

MICRF220

The value of CAGC impacts the time to good data (TTGD),

which is defined as the time when signal is first applied, to

when the pulse width at DO is within 10% of the steady

state value. The optimal value of CAGC depends upon the

setting of the SEL0 and SEL1 pins. A smaller CAGC value

does NOT always result in a shorter TTGD. This is due to

the loop dynamics, the fast discharge current being 600µA,

and the charge current being only 1.5µA. For example, if

VSEL0 = VSEL1 = 0V, the low pass filter bandwidth is set to a

minimum and CAGC capacitance is too small, TTGD will

be longer than if CAGC capacitance is properly chosen.

This is because when RF signal first appears, the fast

discharge period will reduce VCAGC very fast, lowering the

gain of the mixer and IF amplifier. But since the low pass

filter bandwidth is low, it takes too long for the AGC

comparator to see a reduced level of the audio signal, so it

can not stop the discharge current. This causes an

undershoot in CAGC voltage and a corresponding

overshoot in RSSI voltage. Once CAGC undershoots, it

takes a long time for it to charge back up because the

current available is only 1.5µA.

Table 3 lists the recommended CAGC values for different

SEL0 and SEL1 settings.

Figure 3. RSSI Overshoot and Slow TTGD (9.1ms)

Figure 4 shows the behavior with a larger capacitor on

CAGC pin (2.2μF), VSEL1 = 0V, and VSEL0 = VDD. In this

case, VCAGC does not undershoot (RSSI does not

overshoot), and TTGD is relatively short at 1ms.

VSEL1

0V

0V

VDD

VDD

VSEL0

0V

VDD

0V

VDD

CAGC value

4.7μF

2.2μF

1μF

0.47μF

Table 3. Minimum Suggested CAGC Values

Figure 3 illustrates what occurs if CAGC capacitance is too

small for a given SEL1, SEL0 setting. Here, VSEL1 = 0V,

VSEL0 = VDD, the capacitance on CAGC pin is 0.47μF, and

the RF input level is stepped from no signal to −100dBm.

RSSI voltage is shown instead of CAGC voltage because

RSSI is a buffered version of CAGC (with an inversion and

amplification). Probing CAGC directly can affect the loop

dynamics through resistive loading from a scope probe,

especially in the state where only 1.5μA is available,

whereas probing RSSI does not. When RF signal is first

applied, RSSI voltage overshoots due to the fast discharge

current on CAGC, and the loop is too slow to stop this fast

discharge current in time. Since the voltage on CAGC is

too low, the audio signal level is lower than the slicing

threshold (voltage on CTH), and DO pin is low. Once the

fast discharge current stops, only the small 1.5µA charge

current is available in settling the AGC loop to the correct

level, causing the recovery from CAGC undershoot/RSSI

overshoot condition to be slow. As a result, TTGD is about

9.1ms.

Figure 4. Proper TTGD (1ms) with Sufficient CAGC

Reference Oscillator

The reference oscillator in the MICRF220 (Figure 5) uses a

basic Pierce crystal oscillator configuration with MOS

transconductor to provide negative resistance. Though the

MICRF220 has built-in load capacitors for the crystal

oscillator, the external load capacitors are still required for

tuning it to the right frequency. RO1 and RO2 are external

pins of the MICRF220 to connect the crystal to the

reference oscillator.

August 2010

10

M9999-082610-A

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